Method for efficient daily constancy check or calibration of proton therapy system

ABSTRACT

Systems and methods are provided for efficiently performing daily maintenance or quality assurance on proton therapy systems. Specifically, a system is provided which includes a solid-state phantom, a plurality of ionization chambers disposed within the solid-state phantom, and a measuring device coupled to the plurality of ionization chambers and operable to perform radiation measurements of proton beams received within the plurality of ionization chambers. Measurements of proton beams received in the ionization chambers may be used to derive the dose at the ionization chambers and be subsequently compared to pre-generated target data (e.g., data corresponding to proper treatment according to a radiation therapy treatment plan). If the data obtained through the maintenance procedure does not conform to the target data, the proton beam generator may be further calibrated.

TECHNICAL BACKGROUND

Radiation therapy (RT) is a popular and efficient method for cancertreatment, where ionizing radiation is used in an attempt to destroymalignant tumor cells or to slow down their growth. RT is often combinedwith surgery, chemotherapy, or hormone therapy, but may also be used asa primary therapy mode. Radiation therapy may be administered asinternal RT, brachytherapy or, more commonly, external beam RT.

External beam RT typically involves directing beams of radiatedparticles produced by sources located externally with respect to thepatient or subject to the afflicted treatment area. The beam can consistof photons, electrons, protons or other heavy ions. Malignant cells aredamaged by the ionizing radiation used during the RT. However, thedamage from the radiation is not limited to malignant cells and thus,the dosage of radiation to healthy tissues outside the treatment volumeis ideally minimized to avoid being collaterally damaged.

Proton therapy is one type of external beam radiation therapy, and ischaracterized for using a beam of protons to irradiate diseased tissue.The chief advantage of proton therapy over other particle-basedtherapies is the ability to more precisely localize the radiation dosagewhen compared with other types of external beam radiotherapy. Duringproton therapy treatment, a particle accelerator, such as a cyclotron,is used to generate a beam of protons which is subsequently directed ata tumor or target region. As the beam travels through matter (e.g., thesubject), energy from the ionizing radiation is deposited along the pathin the surrounding matter. This energy is known as “dose,” and is usedto measure the efficacy and accuracy of a radiation beam. Conventionalparticle accelerators used for proton therapy typically produce protonswith energies in the range of 70 to 250 MeV (Mega-electron Volts:million electron Volts). As with other radiation therapies, the chargedparticles in proton therapy damage the DNA of cells, ultimately causingtheir death or interfering with their ability to reproduce. Cancerouscells, because of their high rate of division and their reduced abilityto repair damaged DNA, are particularly vulnerable to attack on theirDNA.

Due to their relatively large mass, proton beams typically will havecommensurately less lateral side scatter in the tissue. All protons of agiven energy will operate according to a certain range; and relativelyfew protons will travel (i.e., penetrate) beyond that distance. Inaddition, the radiation dose delivered to the tissue at a proton beam'starget is at its apex during the last few millimeters of the particle'srange—this maximum is referred to as the Bragg peak. To treat tumors atgreater depths, the proton accelerator must produce a beam with higherenergy, typically represented in electron volts (eV). Tumors closer tothe surface of the body are treated using protons with lower energy.Thus, damage from the proton beam may be localized to malignant cells byadjusting the energy of the protons during application of treatment.

The purpose of traditional RT treatment planning methodologies is todevise a treatment regimen which produces as uniform a dose distributionas possible to the target volumes whilst minimizing the dosage outsidethis volume. It is crucial to successful radiation therapy that thediscrepancies between dose distributions calculated at the treatmentplanning stage and those delivered to the patient are minimized.Moreover, just as calculating precise levels of radiation at thetreatment planning stage is of great importance, naturally, so is thesuccess of the application of radiation treatments according to thetreatment plans. Discrepancies between planned treatment dosages andactually administered treatment dosages can lead to unexpected andpotentially disastrous results. Accordingly, proper calibration and(ideally daily) maintenance and quality assurance of the treatmentenvironment, and of the generated radiation beam itself is extremelyvital.

Conventional maintenance or quality assurance procedures often includetests to determine a proton beam's range, and constancy. A typical rangetest may comprise, for example, directing a proton beam according to atreatment plan into an artificial target known as a phantom. Typically,these phantoms are implemented as plastic or glass tanks containingwater with submerged or partially submerged radiation measurementdevices. The phantom is mounted in one or more positions which occupythe iso-center(s) of the proton beam according to a radiation treatmentplan. A proton beam is generated in a particle accelerator according tothe pre-determined treatment plan and received in the radiationmeasurement devices which is scanned while receiving the proton beam torecord the beam's characteristics (e.g., energy). Once the beam'scharacteristics are recorded, the beam may be calculated and compared tothe treatment plan to determine congruency or a lack thereof. The protonbeam generator may be subsequently re-calibrated to eliminate ormitigate any identified discrepancies between the planned beam and theactual tested beam.

Unfortunately, conventional range determination procedures may requireliquid-state phantoms to be quite large. Naturally, due to the volume ofliquid required, the phantoms (particularly when filled) may becumbersome, heavy and/or physically difficult to move and adjust, oftenrequiring two or more operators just to setup the procedure. Inaddition, such maintenance procedures may require significant time torefill and drain each phantom before and after use. Since theseprocedures must (ideally) be performed prior to the actual applicationof radiation treatment, in circumstances where treatments areadministered with great frequency, the additional time, effort andpersonnel required to even attempt to perform such maintenanceprocedures can become quite prohibitive to conduct efficiently.

Another procedure commonly performed during daily clinical maintenanceand quality assurance tests include constancy tests which measure theflux of a field of radiation generated by a proton beam. A typicalconstancy check includes measuring the field of radiation for aplurality of data points using a device with high spatial resolution,such as a film. However, using a medium or device having a high spatialresolution requires processing for each data point. For devices withhigh spatial resolution, this can be an extremely time intensivepractice, especially when coupled with the performance of inefficientrange detection procedures. Moreover, a single layer of film is notreusable, and repositioning multiple layers of film can betime-consuming, especially for frequent or daily tests.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that is further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

A system is provided for efficiently performing daily maintenance orquality assurance on proton therapy systems. Specifically, the systemenables performance of a quick and simple daily quality assurance andmaintenance procedure for proton therapy systems. The system includes asolid-state phantom, a plurality of ionization chambers disposed withinthe solid-state phantom, and a measuring device coupled to the pluralityof ionization chambers and operable to perform radiation measurements ofproton beams received within the plurality of ionization chambers.Measurements of proton beams received in the ionization chambers may beused to derive the dose at the ionization chambers and be subsequentlycompared to pre-generated target data (e.g., data corresponding toproper treatment according to a radiation therapy treatment plan). Ifthe data obtained through the maintenance procedure does not conform tothe target data, the proton beam generator may be further calibrated.

According to one embodiment, the solid state phantom may be composed ofa plastic or similar, relatively light-weight material, and may beconstructed to be of only a few centimeters in depth, thereby allowingeasier mobility and adjustment, without requiring multiple operators ora water source and drain facilities. This novel system allows theperformance of daily maintenance procedures to be performed which areinsensitive to exact alignment of the phantom with respect to theionization chambers, as well as makes use of the distal falloff of theBragg peak of the proton beam by positioning an ionization chamber inthe location of the distal falloff, thereby allowing the identificationof small drifts in the beam's energy and therefore, efficientcalculation of the range of a particle beam.

According to another aspect, a method is provided to perform constancychecks of a scanning system during daily maintenance of a proton beamspot scanning treatment system with a low resolution measurement deviceor medium. According to one embodiment, a proton beam is applied to aplurality of positions (e.g., a single beam may be magneticallyre-directed to a plurality of spots) according to a spot scanningtreatment plan. The beam is received in a re-usable, two dimensionalarray of regularly spaced ionization chambers at a low density. Having alower spatial resolution arrangement allows a reduced processing time,and may be re-positioned and re-used quickly and easily.

According to yet another aspect, a method for calibrating a treatmentplanning system for a proton spot scanning beam delivery system isprovided. In contrast to conventional proton therapy systems whereindose delivery calculated by the treatment planning system must beadjusted for a given plan via output factors, this novel method allows afull calibration of a treatment planning system for proton beam spotscanning for all treatment plans without the need to use output factors.Calibration may be performed by measuring a homogenous radiation fieldgenerated by a proton beam in a simple, yet precise procedure involvinga solid-state phantom with a single thimble ionization chamber disposedin the phantom at a shallow depth. During the testing procedure, thephantom (and ionization chamber) is placed at the iso-center of thetreatment beam, where it receives a proton beam and generates aradiation field from the beam. The field is measured and, given supplieddata in the form of number of radiation treatment spots, spot distancesand monitor units per spot, a dose of the field may be calculated. Thecalculated dose can be subsequently compared to the intended doseaccording to a treatment plan. Unlike conventional, fluence-basedcalibrations, the dose to monitor unit calibrations according toembodiments are not energy dependent, thereby reducing the complexityand uncertainty of such a maintenance procedure.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention:

FIG. 1 depicts an illustration of a conventional liquid-state phantomused during maintenance or quality assurance procedures for radiationtherapy devices.

FIG. 2 depicts an illustration of a solid-state phantom comprising aplurality of ionization chambers used during maintenance or qualityassurance procedures for radiation therapy devices in accordance withembodiments of the present invention.

FIG. 3 depicts an illustration of a solid-state phantom comprising a twodimensional array of ionization chambers used during maintenance orquality assurance procedures for radiation therapy devices in accordancewith embodiments of the present invention.

FIG. 4 depicts an illustration of a solid-state phantom comprising athimble ionization chamber used during maintenance or quality assuranceprocedures for radiation therapy devices in accordance with embodimentsof the present invention.

FIG. 5 depicts a flowchart of a method of performing a range test for aproton therapy system during maintenance or quality assuranceprocedures, in accordance with embodiments of the present invention.

FIG. 6 depicts a flowchart of a method of performing a constancy testfor a proton therapy system during maintenance or quality assuranceprocedures, in accordance with embodiments of the present invention.

FIG. 7 depicts a flowchart of a method of performing a calibration of aproton therapy system based on a calculated radiation dose performedduring maintenance or quality assurance procedures, in accordance withembodiments of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to several embodiments. While thesubject matter will be described in conjunction with the alternativeembodiments, it will be understood that they are not intended to limitthe claimed subject matter to these embodiments. On the contrary, theclaimed subject matter is intended to cover alternative, modifications,and equivalents, which may be included within the spirit and scope ofthe claimed subject matter as defined by the appended claims.

Furthermore, in the following detailed description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe claimed subject matter. However, it will be recognized by oneskilled in the art that embodiments may be practiced without thesespecific details or with equivalents thereof. In other instances,well-known methods, procedures, and components, have not been describedin detail as not to unnecessarily obscure aspects and features of thesubject matter.

Portions of the detailed description that follows are presented anddiscussed in terms of a method. Although steps and sequencing thereofare disclosed in figures herein (e.g., FIGS. 5, 6, and 7) describing theoperations of this method, such steps and sequencing are exemplary.Embodiments are well suited to performing various other steps orvariations of the steps recited in the flowchart of the figure herein,and in a sequence other than that depicted and described herein.

Conventional Liquid-State Phantoms

With reference now to FIG. 1, an illustration of a conventionalliquid-state phantom 100, such as the phantoms typically used during theperformance of certain maintenance procedures for radiation therapydevices (such as proton therapy devices) is depicted, in accordance withone embodiment. In one configuration, the liquid-state phantom 100includes a tank 101 for containing a volume of a liquid 103, and aplurality of radiation monitoring devices 105 submerged or partiallysubmerged within the volume of water 103 and coupled to a detector (notshown) external to the tank 101. Typically, the tank 101 may becomprised of a glass or clear plastic material, and the volume of liquid103 may consist of, for example, water. In one embodiment, the radiationmonitoring devices 105 are arranged such that they do not causeinterference during radiation measurements.

A conventional range test may comprise, for example, mounting aliquid-state phantom 100, pumping in a volume of liquid 103 into thephantom 100, generating a beam of irradiated particles in a particleaccelerator according to a pre-determined treatment plan, directing abeam of irradiated particles into the liquid-state phantom 100, andreceiving the particles in the radiation monitoring devices 105 whichare scanned while receiving the proton beam to record the beam'scharacteristics (e.g., energy at a given range).

However, conventional range determination procedures may requireliquid-state phantoms to be of significant size. For example, at 250 MVenergy, (a common energy value of a generated proton beam) a proton beamcan have a range of up to 38 centimeters. This typically results inusage of liquid-state phantoms which approximate with dimensions of atleast 50×50×50 centimeters, with a corresponding volume of water.Naturally, due to the weight of liquid in the tank 101, the phantoms 100(particularly when filled) may be cumbersome, heavy and/or physicallydifficult to move and adjust, often requiring two or more operators justto setup the procedure.

Exemplary Solid-State Phantom for Performance of Range Tests

According to an embodiment of the present claimed subject matter,quality assurance procedures performed on radiation therapy systems areprovided, including a range test for a beam generated by a radiationtherapy device that may be performed at regular intervals as part of oneor more maintenance procedure(s). With reference now to FIG. 2, asolid-state phantom 200 comprising a plurality of ionization chambers203 used during maintenance procedures (e.g., a range test) forradiation therapy devices is depicted, in accordance with oneembodiment. The solid-state phantom 200 may comprise a solid-statestructure 201, a plurality of ionization chambers 203 disposed withinthe solid-state phantom 200, and a measuring device coupled to theplurality of ionization chambers 203 and operable to perform radiationmeasurements of proton beams received within the plurality of ionizationchambers 203 (not shown). Measurements of proton beam energy received inthe ionization chambers 203 may be used to derive the range at theionization chambers 203, and be subsequently compared to pre-generatedtarget data (e.g., data corresponding to proper treatment according to aradiation therapy treatment plan). If the data obtained through themaintenance procedure does not conform to the target data, the protonbeam generator may be further calibrated.

According to one embodiment, the solid state tank 201 may be composed ofa plastic or similar, relatively light-weight material, such asPolymethyl methacrylate (PMMA) and may be constructed to be of only afew centimeters in depth. In comparison to conventional, liquid-statephantoms, a solid-state phantom according to embodiments of the presentinvention will be both lighter and smaller, thereby allowing easiermobility and adjustment, without requiring multiple operators or a watersource and drain facilities. As depicted, the solid-state phantom 200comprises a total of 5 ionization chambers 203, arranged horizontallyacross a single plane, though embodiments of the present invention arewell suited to alternate implementations comprised of a different numberof ionization chambers, and/or arrangements. In one embodiment, theionization chambers 203 are disposed within the solid-state tank 201 atdifferent depths (from the perspective of an oncoming proton beam), andmay be used to obtain a set of energies at different angles with respectto the gantry of the generating proton therapy device. The ionizationchambers 203 are preferably arranged such that their respectivepositions within the solid-state tank 201 do not cause interferenceduring energy measurements.

As the ionization chambers are positioned in the solid-state phantom ina rigid state, this novel system allows the performance of maintenanceor quality assurance procedures at regular intervals (e.g., daily,weekly, monthly, etc.) to be performed which are insensitive to exactalignment of the phantom with respect to the ionization chambers, aswell as makes use of the distal falloff of the Bragg peak of the protonbeam by positioning an ionization chamber in the location of the distalfalloff, thereby allowing the identification of small drifts in thebeam's energy and therefore, efficient calculation of the range of aparticle beam.

Exemplary Solid-State Phantom for Performance of Constancy Tests

With reference now to FIG. 3, a solid-state phantom 300 comprising atwo-dimensional matrix of ionization chambers 303 used duringmaintenance or quality assurance procedures (such as a constancy test)for radiation therapy devices is depicted, in accordance with oneembodiment. The solid-state phantom 300 may comprise a solid-statestructure 301, and a two-dimensional array of ionization chambers 303disposed within the solid-state phantom 300. In one embodiment, thetwo-dimensional array of ionization chambers 303 corresponds to arelatively low resolution (i.e., relatively fewer amount of datapoints). As shown, a four by four two-dimensional array of ionizationchambers 303 is presented for simplicity in FIG. 3. Alternateembodiments may include two-dimensional arrays of different dimensions.For example, a 27 by 27 (729 total) array of ionization chambers may bepreferable in some embodiments. Naturally, embodiments are well suitedto arrays of various dimensions.

During a constancy test performed with a proton therapy device, a protonbeam is generated and applied to a plurality of targeted areas or“spots” on a target or subject in a pre-defined sequence according to atreatment plan for that particular target or subject. In one embodiment,the plurality of targeted spots form a raster comprising portions or allof the two-dimensional array of ionization chambers. Magnetic fields aregenerated to alter (i.e., by deflecting) the course of the proton beamssuch that the proton beams are directed to each target spot.Measurements of the absorbed doses deposited by the deflected protonbeam and received in the ionization chambers 303 may be used to derivethe energy levels of the proton beam at the ionization chambers 303,where the data may be subsequently compared to pre-generated target data(e.g., DICOM data corresponding to proper treatment according to aradiation therapy treatment plan).

If the data obtained through the maintenance or quality assuranceprocedure does not conform to the target data, the proton beam generatormay be further calibrated such that a constant energy is maintained ateach spot over the entire treatment field, and according to theradiation therapy treatment plan. Since the resolution of the dataobtained over the radiation field is relatively low in comparison totraditional mediums such as film, such a configuration would require asmall fraction of the data processing to process a beam treatmentsequence. Naturally, this could potentially result in drastic decreasein both time and the amount of processing resources required to performfrequent (e.g., daily) constancy checks of a radiation treatment system.

Exemplary Solid-State Phantom for Performance of Monitor Unit to DoseCalculation

With reference now to FIG. 4, a solid-state phantom 400 comprising asingle (thimble) ionization chamber 403 used during maintenance orquality assurance procedures (such as dose to monitor unit calculation)for radiation therapy devices is depicted, in accordance with oneembodiment. The solid-state phantom 400 may comprise a solid-statestructure 401, and a thimble ionization chamber 403 disposed within thesolid-state phantom 400 at a shallow depth d (e.g., 1-2 cms). In oneembodiment, the solid-state phantom 400 is mounted such that theionization chamber 403 corresponds to the iso-center of a radiation beamtreatment according to a radiation therapy plan.

During a dose to monitor unit calculation test performed with a protontherapy device, a proton beam is generated and applied to a plurality oftargeted areas or “spots” on a target or subject in a pre-definedsequence according to a treatment plan for that particular target orsubject. During maintenance tests, the target or subject is simulatedwith a solid-state phantom, such as the phantom 400 depicted in FIG. 4,according to some embodiments. A homogenous, mono-energetic calibrationfield is generated with pre-defined properties (such as the field sizeat iso-center, distance between spots, and monitor units per spot).Given the homogenous calibration field, the dose (and, by extension, theabsorbed dose amount per number of monitor units) may be measured withinthe ionization chamber.

The measured dose can be subsequently utilized to normalize (andcalibrate) the beam parameters in a treatment plan by comparing themeasured dose with expected or target doses. By utilizing this systemwhich incorporates a solid-state phantom, fluence deviationsattributable to liquid-state phantoms may be avoided. The shallowcalibration depth and homogenous, mono-energetic calibration fieldavoids dose gradients in a Bragg peak, thereby advantageously providingrobust calibration results independent of setup particularities.

Performing a Range Test on Proton Therapy Device

FIG. 5 is a flowchart 500 depicting a method for performing a range testwith a solid-state phantom on a radiation therapy device (e.g., a spotscanning proton therapy device), in accordance with one embodiment.Specifically, the method enables the simple and efficient calculation ofthe range of an emitted radiation beam during a maintenance process of acorresponding radiation therapy device. Steps 501-507 describe exemplarysteps comprising the process depicted in flowchart 500 in accordancewith the various embodiments herein described. In one embodiment, atleast a portion of the steps described flowchart 500 may be performed ascomputer-executable instructions stored in a computer-readable medium.

At step 501, a solid-state phantom is mounted in a pre-determinedposition with respect to a proton therapy device. In one embodiment,mounting the solid-state phantom comprises positioning the phantom at adesired distance and/or at a desired angle with respect to a gantry ofthe proton therapy device. In some embodiments, the solid-state phantommay comprise the plastic or PMMA phantom 200 described above withrespect to FIG. 2. At step 503 a proton beam (e.g., a pencil beam) isgenerated in a particle accelerator and applied to a solid-state phantomto generate a two dimensional mono-energetic field of protons during theperformance of a maintenance test. The proton beam may be applied by,for example, generating a beam of protons in a particle accelerator andgenerating a magnetic field to direct the proton beam to desiredtargeted positions in the solid-state phantom.

In one embodiment, the desired positions in the solid-state phantom 200correspond to the positions of the plurality of ionization chambersdisposed within the solid-state phantom 200. As the proton beam travelsthrough the solid-state phantom 200, energy (i.e., the “dose” of theproton beam) is deposited in the material of the solid-state phantom 200along the path of the proton beam. The dose of the proton beam may bemeasured and recorded within the ionization chambers 203. Thus, the doseof the proton beam at the positions in the field corresponding to thepositions of the ionization chambers 203 may be measured and recorded.According to some embodiments, the size of the mono-energetic field ofprotons generated at step 503 may be substantially equivalent to thevolume of the solid-state phantom 200, such that the solid-state phantom200 may be substantially covered by the mono-energetic field of protons.

At step 505, the doses of the mono-energetic field of protons absorbedby the plurality of ionization chambers 203 within the solid-statephantom 200 are recorded as data. Ionizing radiation (such as radiationfrom proton beams) experience energy loss as a result of travelingthrough matter. This energy loss, commonly plotted as a Bragg curve—alsoexhibits a pronounced peak immediately before the particles come to rest(e.g., where the energy of the particle beam exhibits a significantreduction). This reduction is referred to as the distal falloff theBragg Peak. For protons, a pronounced peak, called the Bragg peak, isexhibited in a Bragg curve immediately before the protons of a protonbeam come to rest, where the energy of the proton beam exhibits asignificant reduction (the “distal falloff”). According to someembodiments, the last chamber of the plurality of ionization chambers203 may be positioned to correspond to the distal falloff the Bragg Peakfor a proton beam according to a radiation therapy plan. Accordingly,dose deviations in the remainder of the plurality of ionization chambers203 may be attributed to deviations from the dose measurement. Thiseliminates the need to account for the distal falloff when calculatingdose which is required by typical range tests, due to the relative lackof mobility of a liquid-state phantom.

At step 507, the measured doses recorded in the plurality of ionizationchambers at step 505 may be compared to pre-determined reference data todetermine the need for proper (or additional) calibration of theradiation therapy device. In some embodiments, the pre-determinedreference data comprises data under the digital imaging andcommunications in medicine (DICOM) standard. Reference data may compriseplotted data calculated according to a radiation treatment plan.Reference data may also comprise, for example, measured data fromprevious range tests. By incorporating a solid-state phantom 200, arange test may be performed on a radiation therapy device efficientlyand effectively, without requiring multiple operators and whileeliminating the need to perform (now) extraneous accounting for distalfalloff of the proton beam.

Performing a Constancy Test on Proton Therapy Device

FIG. 6 is a flowchart 600 depicting a method for efficiently performinga constancy test with a solid-state phantom on a radiation therapydevice (e.g., a spot scanning proton therapy device), in accordance withone embodiment. Specifically, the method enables the simple andefficient calculation of the constancy of a homogenous flux of radiationgenerated at a target from a radiation beam emitted during a maintenanceprocess of a corresponding radiation therapy device. Steps 601-611describe exemplary steps comprising the process depicted in flowchart600 in accordance with the various embodiments herein described. In oneembodiment, at least a portion of the steps described flowchart 600 maybe performed as computer-executable instructions stored in acomputer-readable medium.

At step 601, a proton beam (such as a pencil beam) is applied to atwo-dimensional array of ionization chambers 303 during a maintenance orquality assurance test. In one embodiment, the pencil beam may beapplied to a plurality of positions corresponding to individualionization chambers within the two-dimensional array of ionizationchambers 303. These positions may correspond to the planned treatmentspots of a treatment plan (or pre-defined test plan). According to someembodiments, the two-dimensional array of ionization chambers 303 may bedisposed in a solid-state phantom 301 (e.g., the solid-state phantom 300described above with respect to FIG. 3), mounted in a pre-determinedposition with respect to a proton therapy device. According to oneembodiment, the dimensions of the solid-state phantom 301 may correspondto the dimensions of conventional, liquid containing phantoms. Accordingto further embodiments, the solid-state phantom 301 may be any sizesuitable to contain the two-dimensional array of ionization chambers303.

In one embodiment, the two-dimensional array of ionization chambers 303may comprise a 27 by 27 arrangement (729 total single ionizationchambers) of single ion chambers separated from each other by, forexample, a distance of approximately one centimeter, thereby providing arelatively low-resolution apparatus capable of measuring the dose of thepencil beam over a larger volume. Naturally, embodiments are well suitedto other arrangements, and in particular, to arrangements maintaininglow-resolution/wide volume coverage. As with the solid-state phantomused in the range test described above with respect to FIG. 4, thesolid-state phantom may also be constructed from plastic or PMMA.

At step 603 the proton beam generated and applied at 601 is received ina first location in the solid-state phantom 301 according to apre-determined treatment plan (or pre-defined test plan) andcorresponding to a first ionization chamber in the two-dimensional arrayof ionization chambers 303. At step 605, the proton beam is redirectedto a second location in the solid-state phantom 301 corresponding to asecond ionization chamber of the two-dimensional array of ionizationchambers 303 where it is received at step 607. The second location may,for example, correspond to the next target spot in a sequence of targetspots according to the pre-determined treatment plan. In one embodiment,the proton beam is directed by generating a magnetic field through acurrent at the treatment head of the radiation therapy device.Redirection of the proton beam performed at step 605 may be performed byaltering the magnetic field to influence the path of the emitted protonbeam. Steps 603 to 607 may be repeated for additional spots according tothe treatment plan.

At step 609, the doses deposited by the pencil beam within theionization chambers of the two-dimensional array of ionization chambers303 during execution of the treatment plan are recorded and measured. Atstep 611, the measured doses recorded in the two-dimensional array ofionization chambers 303 at step 609 may be compared to pre-determinedreference data to determine the need for proper (or additional)calibration of the radiation therapy device. In some embodiments, thepre-determined reference data comprises data under the digital imagingand communications in medicine (DICOM) standard. Reference data maycomprise plotted data calculated according to a radiation treatmentplan. Reference data may also comprise, for example, measured data fromprevious constancy tests. By incorporating two-dimensional array ofionization chambers with a relatively low-resolution in a solid-statephantom 200, a constancy test may be performed on a radiation therapydevice efficiently and effectively, without requiring multiple operatorsand while drastically reducing the amount of processing required forconventional constancy tests conducted over traditional mediums.

Performing Monitor Unit to Dose Calibration

FIG. 7 is a flowchart 700 depicting a method for efficiently performinga calculation of dose generated from measurements of detected monitorunits observed during the course of a radiation therapy treatment. Thistherapy may include, for example, proton spot scanning treatments whichapply a pencil beam of protons to a plurality of target spotsdistributed over a target treatment area. When the proton beam isreceived in an ionization chamber, such as during maintenance or qualityassurance tests, the air in the dose monitor chambers of the radiationtreatment head through which the proton beam passes become ionized withenergy (the dose of the proton beam). The ionized air creates chargesknown as “monitor units” which can be measured and recorded, from whicha dose corresponding to the proton beam may be subsequently derived.Steps 701-711 describe exemplary steps comprising the process depictedin flowchart 700 in accordance with the various embodiments hereindescribed. In one embodiment, at least a portion of the steps describedflowchart 700 may be performed as computer-executable instructionsstored in a computer-readable medium.

At step 701, a proton beam (such as a pencil beam) is generated during amaintenance or quality assurance test. At step 703, the generated protonbeam is received in a calibrated ionization chamber disposed in a solidstate phantom. In one embodiment, the proton beam comprises a pencilbeam generated from a proton therapy device, and the ionization chambercomprises a thimble ionization chamber disposed at a relatively shallow(approximately 1-2 cm) depth in the solid state phantom, such as thesolid-state phantom 400 described above with respect to FIG. 4. Infurther embodiments, the phantom 400 may be mounted at a pre-determinedposition such that the position of the thimble ionization chamber 403corresponds to the iso-center of a radiation therapy treatment plan.According to still further embodiments, the pencil beam may be appliedto a plurality of positions within or about the phantom 400 to generatea homogenous radiation field. The homogenous radiation field may begenerated with pre-determined characteristics, such as the field size atthe iso-center, number of spots, the distance between spots, and monitorunits per spot, etc. In further embodiments, the homogenous radiationfield generated is a mono-energetic, two-dimensional radiation field.

The homogenous radiation field is measured at 705 and compared to a setof pre-defined proton beam data at step 707 to determine the doseabsorbed in the thimble ionization chamber 403 during the maintenance orquality assurance test. Given the pre-determined characteristics of thegenerated homogenous radiation field, the dose may be calculated at step709 by, for example, comparing the absorbed dose observed in step 705 atthe thimble ionization chamber 403 during the test with thepre-determined beam field characteristics referenced at step 707.According to one embodiment, calculating the dose may be performed by,for example, deriving the ratio from dividing the dose by the number ofmonitor units of the homogenous radiation field generated in step 701for each data point in the set of data. Finally, at step 711, the dosemeasured in step 709 can be subsequently utilized to normalize (andcalibrate) the parameters of the proton beam in a treatment plan bycomparing the measured dose with expected or desired doses.

As described herein, systems and methods for performing dailymaintenance or quality assurance of a proton therapy system whichdrastically reduce the difficult and/or complexity of conventional dailymaintenance or quality assurance tests. In particular, the usage of asolid-state phantom to replace traditional liquid-state phantomseliminate a significant portion of user effort required to even set upmaintenance tests for range, constancy, and dose calculation, whileproviding robust measurements independent of set up errors. Likewise,the utilization of a two-dimensional array of ionization chambers torecord data drastically reduces the processing required to perform aconstancy test over traditional methods.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed is:
 1. A method to calibrate a proton spot scanning beamdelivery system, the method comprising: receiving, in a plurality ofionization chambers disposed in a solid-state phantom, a portion of aproton beam emanating from a radiation treatment head and traversing atleast a portion of plurality of ionization chambers, the proton beambeing directed at the plurality of ionization chambers according to aradiation plan; detecting, via at least one ionization chamber of theplurality of ionization chambers, a plurality of monitoring unitsgenerated in response to charged air particles produced from a passageof the proton beam through the at least one ionization chamber;measuring a radiation field generated from passage of the proton beamthrough the portion of the plurality of ionization chambers; calculatinga radiation dose corresponding to the measured radiation field based ona dose absorbed by the plurality of ionization chambers and the numberof monitoring units generated from the passage of the proton beam;comparing the measured radiation field to the set of pre-defined protonbeam data, the set of pre-defined proton beam data comprising a targetradiation dose; and calibrating the proton spot scanning beam deliverysystem to generate a proton beam based on the target radiation dose. 2.The method according to claim 1, wherein the radiation field comprisesat least one of: a two-dimensional radiation field; and a homogeneousradiation field.
 3. The method according to claim 1, wherein theradiation field is a mono-energetic radiation field.
 4. The methodaccording to claim 1 wherein measuring the radiation field to determinean absorbed dose comprises measuring an absorbed dose per number ofmonitor units from receiving the proton beam.
 5. The method according toclaim 1, wherein the ionization chamber is a thimble ionization chamber.6. The method according to claim 1, wherein the ionization chamber isdisposed within the solid-state phantom at a shallow depth.
 7. Themethod according to claim 1, wherein the ionization chamber is disposedwithin a liquid phantom at a shallow depth.
 8. The method according toclaim 1, wherein the proton beam comprises a pencil beam.
 9. The methodaccording to claim 1, wherein the set of pre-defined proton beam datacomprises at least one data type from the group comprised of: number oftarget spots, spot distance, and monitor units per spot.
 10. The methodaccording to claim 1, wherein the plurality of ionization chambers arecomprised in a matrix configuration in the solid-state phantom.
 11. Themethod according to claim 1, further comprising at least one of:measuring the plurality of monitoring units corresponding to theplurality of ionized air particles produced from the passage of theproton beam through the at least one ionization chamber; and recordingthe monitoring units corresponding to the plurality of ionized airparticles produced from the passage of the proton beam through the atleast one ionization chamber.
 12. The method according to claim 11,further comprising calculating a dose received in the plurality ofionization chambers by determining the ratio of a dose of the protonbeam with a number corresponding to the plurality of monitoring units.13. The method according to claim 1, wherein a last ionization chamberof the plurality of ionization chambers is positioned to specificallycorrespond to a distal falloff of the Bragg Peak for the proton beamaccording to a radiation therapy plan.